"No, no," Gates said in a joint appearance with Secretary of State Hillary Clinton, their first since the Libya operation began. "It was not -- it was not a vital national interest to the United States, but it was an interest and it was an interest for all of the reasons Secretary Clinton talked about. The engagement of the Arabs, the engagement of the Europeans, the general humanitarian question that was at stake."

Gates explained that there was more at stake. "There was another piece of this though, that certainly was a consideration. You've had revolutions on both the East and the West of Libya," he said, emphasizing the potential wave of refugees from Libya could have destabilized Tunisia and Egypt.

"So you had a potentially significantly destabilizing event taking place in Libya that put at risk potentially the revolutions in both Tunisia and Egypt," the Secretary said. "And that was another consideration I think we took into account."

During his campaign for the presidency, in December, 2007, Barack Obama told The Boston Globe that "The president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."

Earlier in 2007, then-Senator Hillary Clinton said in a speech on the Senate floor that, "If the administration believes that any -- any -- use of force against Iran is necessary, the President must come to Congress to seek that authority."

Tapper asked Clinton, "Why not go to Congress?"

"Well, we would welcome congressional support," the Secretary said, "but I don't think that this kind of internationally authorized intervention where we are one of a number of countries participating to enforce a humanitarian mission is the kind of unilateral action that either I or President Obama was speaking of several years ago."

"I think that this had a limited timeframe, a very clearly defined mission which we are in the process of fulfilling," Clinton said.

Earlier in the interview, which was taped on Saturday afternoon, Gates extrapolated the White House's timeline on Libya.

"Some NATO officials say this could be three months, but people in the Pentagon think it could be far longer than that. Do you think we'll be gone by the end of the year? Will the mission be over by the end of the year?" Tapper asked

"I don't think anybody knows the answer to that," Gates said.

The United States has been at war in Afghanistan for almost ten years, at war in Iraq for almost eight years and at war in Libya for nine days.

"For all practical purposes, the implementation of a no-fly zone is complete," Gates said. "Now it will need to be sustained, but it can be sustained with a lot less effort than what it took to set it up."

On the humanitarian side, the defense secretary said significant progress has been made.